The present invention relates to a plate of a magnetic circuit in a loudspeaker.
In an electrodynamic loudspeaker, which is widely used, an electric signal is converted into an acoustic signal by magnetic flux generated by a magnet and by the magnetic force caused by the electric signal.
Referring to FIG. 1, a cone loudspeaker, which is one type of electrodynamic loudspeaker, has a yoke 12 with an integral center pole 11 and a yoke base 13, an annular magnet 14 mounted on the yoke base 13, and an annular plate 15 mounted on the magnet 14, thereby forming a magnetic circuit. On the plate 15 is mounted a conical frame 20 which supports a conical diaphragm 23 around an upper edge thereof at an edge 21. A lower edge of the diaphragm 23 is disposed in a magnetic gap G formed between the center pole 11 of the yoke 12 and the plate 15 and secured to a voice coil 22 which is supported by a damper 24. The voice coil 22 is disposed in the magnetic gap G to be moved in the direction shown by arrows a and b. A lead 30 connects the voice coil 22 to an external terminal 31 which is connected to an external source to feed audio current to the voice coil 22. When applied with the audio current, the voice coil 22 generates a magnetic force, which moves the voice coil 22 itself. The diaphragm 23 is thus caused to vibrate to produce sound waves.
In the speaker, the base 13 has a uniform thickness as shown in FIG. 1. In such a speaker there is a large leakage flux, which causes the magnetic flux generated in the magnetic gap G to reduce. Japanese Utility Model Publication No. 46-8272 discloses a yoke where the thickness is reduced toward the peripheral portion thereof to prevent the reduction of the magnetic flux.
The inventors of the present invention have proposed a magnetic circuit in Japanese patent application no. 2-280773 (U.S. Ser. No. 666,763) which is shown in FIG. 2. A yoke 12a and a plate 15a are slanted toward the periphery. The upper surface of the plate 15a has an inwardly curved sectional shape 17 and the lower surface of a yoke base 13a has also a curve 16 so that each of the peripheral portions thereof is extremely thinned down. A loudspeaker having such an arrangement is advantageous in that the magnetic flux flows mainly through the gap G so that a leakage flux .phi.1 between the plate 15a and the yoke base 13a, detouring the magnet 14, is decreased. A leakage flux .phi.2 which flows from the plate 15a to the center pole 11 is also decreased.
At the same time, the circuit becomes thinner and lighter in weight.
Japanese Patent Application Laid-open No. 3-155777 also proposed by the inventors discloses a method for manufacturing a plate by cold forging. In the method, a die having a draft on the periphery thereof is used for producing the plate, thereby ensuring the dimensional accuracy of the plate. Thus, the plate having a thin peripheral portion is manufactured with accurate dimensions, and the magnetic circuit having the intended characteristic in the design of the speaker is obtained.
The plate 15 of FIG. 1 is secured to a base portion of the frame 20 with adhesive or bolts. However, the plate 15a of FIG. 2 has the surface inwardly curved without a flat portion. Consequently, it is difficult to secure the frame 20 to the plate 15a.
In order to secure the frame 20 to the curved plate 15a with screws, threaded holes are formed in the base portion of the frame and the plate. Furthermore, in order to form a horizontal surface on the plate 15a, it is necessary to provide an element which has a surface corresponding to the inclined surface of the plate, thereby causing an increase in the number of parts.